Serum markers hold great promise for improving the care and treatment of cancer patients. Although many proteins have serum levels associated with various cancers, each has limited clinical usefulness when measured individually at single time points. The lack of sensitivity and specificity of current serum markers stems from heterogeneity in the baseline levels of the marker proteins and heterogeneity in the tumors and patients. A biomarker discovery strategy that accounts for the heterogeneity in people and tumors is to use individualized thresholds, based on longitudinal measurements, to precisely define abnormal levels for each individual. Previous research has shown that biomarkers defined by longitudinal measurements could have greatly improved specificity and sensitivity over current markers. No systematic study of this topic has been performed, largely because of the lack of a convenient technology for that purpose. A well developed and validated antibody microarray technology in the laboratory of Dr. Haab now makes this exploration possible. The multiplex detection capability of the antibody microarray will allow us to test the performance improvement upon using longitudinal measurements for many different proteins and to establish the general principles that define the use of longitudinal markers. In addition, the multiplex detection capability ultimately will allow the use of combined longitudinal measurements for even further biomarker performance improvement. To test this strategy, we will evaluate the sensitivity, specificity and time of the detection of prostate cancer recurrence using both longitudinal and single-time-point measurements of many different prostate cancer-related proteins in serum. This new approach addresses fundamental issues in biomarker research and should result in valuable information for a wide variety of research areas. The successful demonstration of the approach to prostate cancer diagnostics will signal its potential usefulness for all types of biomarker studies. [unreadable] [unreadable] [unreadable]